Lots of experience in several martial arts and different types (striking, grappling and weapons).

The younger you are, the more you benefit from what I like to call the "basics" which is essentially martial arts that condition the body. The older you are, the more risky and more effort required to benefit from the condition-heavy martial arts.

So, with that in mind, given your young age, if your goal is to,

gain strength - grappling arts require strength and condition many connective tissues and joints. Also, they give you a superb center of balance. Note that if you want to maximise your strength potential (and appearance), you should be prepared to also hit the weights - not the machines though - at your local gym.

gain flexibility and speed - striking arts are generally excellent for increased (even maximum) flexibility and movement speed. Flexibility is extremely undervalued in both the martial arts and sports worlds. It is very useful for preventing many types of injuries, no matter the martial art or sport. It also one of the few things that translates across arts and sports very well, helping you learn new physical techniques more easily. Also, if you practice it early in your life the benefits last you your entire life.

self defence - in general, any martial art (even Tai Chi) will give you both the self-confidence and core self-defense skills that would be more useful in many situations than if you had no martial arts training. This is because your opponents will almost certainly not have any of that training, otherwise they would never risk using it to start fights. That said, you should not have a false sense of security, regardless of what art(s) you chose - even the so called "self defence" ones - because many self-defense situations involve ambushes and learning to avoid these is much more important than any fighting techniques. I will say that in my experience, non-grappling arts are a little more useful for self-defence because all fights start standing and distance management is very important (e.g. opponents may have weapons) and worth learning early.

weapons - there are a surprisingly large number of martial arts that teach weapons of various kinds. However, if they are not dedicated weapons arts, they are mainly based on either fixed, stylized patterns which do not really teach the weapon but illustrate the core art's techniques and develop other core skills, or they use weapons as part of either self-defence or advanced disarming combat. So, if you really want to learn weapons, you are far better off with dedicated weapon arts like Fencing or Kendo. Those really do teach effective weapon use and also develop some very strong movement skills.

achievements/"black belt" - keep in mind, that in general becoming an advanced grappler takes significantly longer than becoming a strong striker or even a weapons master. So, if your goal is to "achieve" something for your resume/CV etc, striking arts are generally the easiest. Of these, the *traditional* Chinese arts (if you're lucky to find a decent school) are generally tougher than the Japanese or Korean arts because they put many demands on many skills at the same time, especially flexibility. The younger you are at starting a Chinese art, the easier it is. Also, not every Chinese art has clear progression like belts, which can be discouraging for a young Western person.

sparring - not every school or martial art emphasizes practical use of technique via sparring. So, if you have strong expectations to be able to spar every lesson, you should be very careful to ask about it and watch several lessons at schools. Some people are put off by too much emphasis on pure technique or core skills all the time and not enough actual "fighting". By contrast, note that many grappling and weapon arts and schools practice a form of fighting in every lesson, especially Judo (called "randori").

At your age, I would recommend keeping away from Aikido and other soft arts which can be learned at any time in your life without serious obstacles. Also, if you have never done a traditional martial art, you will gain a LOT, especially in discipline, respect and strong structure of these old schools - there is far less risk you are being taught crap, dropping out and therefore wasting your time.

If I were in the same situation as you, I would be looking to one striking art *and* one grappling art. Only later would I pick a weapons art and after that a soft art. If I could not do both a striking art and a grappling art at the same time, I would do just a striking art at this time. If neither are available, I would do a weapon art like Fencing or a sport like Muay Thai or Boxing - these are always available.

I would pick arts with the strongest structure as possible, so I could learn continuously, quickly and confidently no matter where I went, even if I changed city or country, e.g. to go university. For striking, this tends to be large and popular Karate or Tae Kwon Do schools, or if you're lucky a Hapkido school. If you are already flexible or want to develop your flexibility strongly, then a good Wushu or Shao-Lin or Sanshou/Sanda school would be an alternative, but they are never as easy to find or transferrable as the Japanese and Korean schools.

For grappling, since you are in Europe, Judo is virtually universal and the standard of training very high and uniform. If not Judo, then either gi-based Brazilian Jiu Jitsu or gi-based Japanese Jiu Jitsu or kurkta-based Sambo. If none of them are available, then a Wrestling school, which are typically Greco-Roman, though you may be lucky and find a Catch Wrestling school.

Also, try to remain far away from any "MMA" school, unless you have no other option, if you have never achieved a high level in any single traditional martial art. The lack of structure and minimum core skill development will slow you down in the long run (unless possibly your aim is to become a professional fighter as fast as possible, in which case better pick a damn good MMA school!), especially as you move around in your young life. Later you can return to these type of schools when you have a solid foundation you can mould around.

At the end of the day, martial arts is a lifestyle, so never fear trying anything new, just be aware it does pay a lot more to become very good in at least one style, rather than hopping from one thing to another. Your journey has just started!